1. Field of the Invention
This invention relates to production and regeneration of concentrated basic (alkaline) hydrogen peroxide by electrosynthesis of water and oxygen in an aqueous solution of potassium hydroxide for the purpose of operating a chemical oxygen-iodine laser.
2. Description of the Related Art
Chemical oxygen-iodine laser (COIL) derives its power from continuous reaction of basic (alkaline) hydrogen peroxide and chlorine to produce electronically excited oxygen known as singlet delta oxygen O.sub.2 (.sup.1 .DELTA.) via reaction EQU Cl.sub.2 +2O.sub.2 H.sup.- (aq)+2K.sub.+ .fwdarw.H.sub.2 O.sub.2 +2KCl+O.sub.2 (.sup.1 .DELTA.) (1)
The singlet delta oxygen is then used to excite iodine atoms to a laser transition. The applicant's co-pending patent application, Ser. No. 09/020,996, filed on Feb. 9, 1998 now abandoned, which is hereby made a part hereof and incorporated herein by reference, teaches how COIL can be operated in conjunction with an electrochemical cell which regenerates products of the basic hydrogen peroxide reaction with chlorine into fresh basic hydrogen peroxide and chlorine reactants. The process uses a porous, packed bed, self-draining, gas diffusion cathode to generate basic hydrogen peroxide by reduction of oxygen in alkaline electrolyte.
Cathodic reduction of oxygen for production of hydrogen peroxide according to the process: EQU O.sub.2 (g)+2H.sub.2 O+2e.sup.- .fwdarw.O.sub.2 H.sup.- (aq)+OH.sup.- (aq)(2)
has been known since the 19th century. However, commercialization of the process has been retarded by several factors related to the complex electrochemistry of oxygen reduction, together with poor understanding of the influence of electrode materials and cell design on process efficiency. Prospects for commercial utilization of the alkaline hydrogen peroxide produced by this reaction as a bleaching agent in the pulp and paper industry motivated the research and development in the last two decades. During that period a number of patents have been awarded for process design, electrode design, configuration of the cell and method of operation.
Hydrogen peroxide generally used in bleaching is in the form of a stabilized alkaline solution of low peroxide concentration, typically 2 to 5% by weight. For economical reasons the alkali metal used is sodium. The relative molar concentration of NaOH to peroxide H.sub.2 O.sub.2 in the bleach solution is generally between 1.0 and 2.0.
Cells with packed bed electrodes are known for Oloman et al U.S. Pat. Nos. 3,969,201 and 4,118,305. Improvements in these cells have been disclosed by McIntyre et al in U.S. Pat. Nos. 4,406,758; 4,431,494; 4,445,986; 4,511,441; and 4,457,953. Packed bed cathodes constructed from graphite chips coated with carbon black and polytetrafluorethylene as a binder have been found particularly suitable for the reduction of oxygen to alkaline hydrogen peroxide. Graphite is a good electrocatalyst, since it is electrically conductive, inexpensive, and requires no special treatment. The hydrophobic nature of the composite chips helps to prevent the cathode from becoming flooded by electrolyte. Moreover, composite chips have an improved capability to handle the flow of electric current in a packed bed. This both prolongs cathode bed life and improves its performance. A method for manufacture such composite particles has been disclosed by McIntyre in U.S. Pat. No. 4,457,953.
Dong in the U.S. Pat. No. 4,891,107 and U.S. Pat. No. 4,921,587 and Mathur in the U.S. Pat. No. 4,927,509 teach that a packed bed, self-draining cathode for maximum productivity within an electrochemical cell for the production of hydrogen peroxide in a solution of sodium hydroxide must be supplied with a liquid anolyte through a porous diaphragm at a substantially uniform rate of flow across the porous diaphragm without appreciable variation of the flow as a function of the head of the electrolyte. Said diaphragm can also be used to control the flow rate of electrolyte into the porous, packed bed cathode so as to avoid flooding the cathode or starving it of electrolyte.
The innovations disclosed by McIntyre, Dong and Mathur, listed above, have been recently utilized by Dow Chemical Company for a construction and operation of a pilot plant for commercial production of solution of hydrogen peroxide and sodium hydroxide intended for use as a bleach in the pulp and paper industry. This plant which is located at Fort Saskatchewan, Alberta, Canada has a capacity of 0.5 ton of H.sub.2 O.sub.2 per day and has been operated continuously for several years. A larger plant with a capacity of 3.5 of H.sub.2 O.sub.2 ton per day plant was built by Dow and installed at the Fort Howard Corporation mill in Muskogee, Okla.
Design attributes and method of operation of the Dow's alkaline peroxide cell are disclosed in the U.S. Pat. No. 4,927,509. At ambient temperature, with a feedstock of 4% by weight (1 mol/liter) of NaOH in water, and current density of approximately 0.3 amperes/in2, the Dow's cell produces alkaline hydrogen peroxide with approximately 4-5% H.sub.2 O.sub.2 concentrations by weight at 80-85% current efficiency. At these concentrations and with the NaOH/H.sub.2 O.sub.2 molar ratio between 1.4 and 1.8 the Dow's cell product is suitable for use as a bleach in the Kraft paper making process. An alternate process used by the pulp & paper industry known as the mechanical pulp process requires a bleach solution with reduced alkalinity. Suitable de-alkalinizer cells which could post-process the output of Dow's cell have been disclosed by Clifford et al in U.S. Pat. No. 5,106,464 and U.S. Pat. No. 5,244,547, and Paleologou et al in U.S. Pat. No. 5,006,211.
Methods for direct production of low alkalinity H.sub.2 O.sub.2 in aqueous solution of NaOH in multi-compartment cells have been disclosed by Kuehn et al in U.S. Pat. No. 4,357,217 and Jasinski et al in U.S. Pat. No. 4,384,931. Both Kuehn and Jasinski claim to produce moderate concentrations of about 8% H.sub.2 O.sub.2 in a low alkalinity solution. However, this result was accomplished at the expense of a high cell voltage (3-7 V), a low current density (0.25 amperes/in.sup.2) and limited current efficiency making the process economically unattractive. The apparent source of problem with Kuehn's and Jasinski's cells was a poor performance of their gas diffusion cathode. Most recently, Dong et al, in the U.S. Pat. No. 5,643,437 discloses a method for co-generation of ammonium persulfate anodically and alkaline hydrogen peroxide cathodically with a cathode product ratio control. While this method can produce a lower alkalinity hydrogen peroxide in 5-6% concentration by weight, operating parameters of the cell, namely the high potential of 5 volt and low current density of 0.1 ampere/in.sup.2 together with low current efficiency of 47% make this process less attractive.
In summary, all of the above processes for manufacturing of basic hydrogen peroxide used sodium cations in the electrolyte and produced only low concentrations of H.sub.2 O.sub.2, typically less than 5% concentration by weight, all operate only at low current densities of typically less than 0.3 amperes/in.sup.2 and, with the exception of Dow's process, all suffered of low current efficiency.
In order to make the electrosynthesis of H.sub.2 O.sub.2 cost competitive with respect to the traditional methods of H.sub.2 O.sub.2 production such as anthraquinone method, the prior art considered the use of NaOH rather than KOH in the cathode electrolyte. However, the use of NaOH makes the cathode susceptible to formation of Na.sub.2 O.sub.2. 8H.sub.2 O (octohydrate) precipitate which gradually plugs the cathode pores (see Jan Balej, Application of Phase Diagram of the System NaOH--H.sub.2 O.sub.2 --H.sub.2 O for the production of Hydrogen peroxide by Cathodic Reduction of Oxygen in Sodium Hydroxide Solutions, collection of Czechoslovak Chemical Communications, vol. 37, p 2830, 1972). Certain advantages of using KOH in peroxide production by electro-reduction of oxygen were observed in early prior art and disclosed by Berl in the U.S. Pat. No. 2,000,815. Berl claimed to have produced peroxide concentration of 5% by weight but at low current density of 0.3 amperes/in2 and with low efficiency of 66%. In another experiment, Berl's cell generated a product of containing a 18% by weight H.sub.2 O.sub.2 and 38% by weight KOH at unspecified, but assumingly low current efficiency.
The basic hydrogen peroxide used in COIL (Chemical Oxygen-Iodine Laser) is an aqueous electrolyte containing hydrogen peroxide, potassium hydroxide, and often also potassium chloride. Experience shows that the basic hydrogen peroxide composition can significantly influence the efficiency and, therefore, the power output of the COIL. In particular, basic hydrogen peroxide with concentrations of H.sub.2 O.sub.2 and KOH each below 2 mols per liter result in excessive quenching of the excited singlet oxygen in the basic hydrogen electrolyte. Similarly, basic hydrogen peroxide with molar ratio of KOH with respect to H.sub.2 O.sub.2 in excess of 1.0 would contain a population of OH.sup.- anions which feeds a parasitic reaction between chlorine and OH.sup.- which does not produce the excited singlet delta oxygen. In either case the supply of the energetic singlet oxygen to the laser is reduced which has the consequential effect of reducing laser power. Furthermore, in order to maintain economical operation, the cell which regenerates basic hydrogen peroxide for the COIL should have a high current efficiency and low voltage. A regeneration cell with high current density is favored as it renders itself to a smaller hardware package. In order to avoid thermal decomposition of hydrogen peroxide the regeneration cell should operate at low temperature, preferably in the vicinity of 0.degree. Centigrade. This is particularly important as no stabilizers can be added to the BHP due to a potential interference with the singlet delta producing reaction. Finally, the singlet delta oxygen producing reaction in Equation 1, above, also consumes chlorine. It is, therefore, desirable for some configurations of the chemical oxygen-iodine laser to combine production of basic hydrogen peroxide and chlorine into one electrolytic cell.
In summary, a suitable electrolytic cell for production of basic hydrogen peroxide by electro-reduction of oxygen for a COIL should 1) produce at least 10% H.sub.2 O.sub.2 by weight and 11% KOH by weight, preferably in a molar ratio of not exceeding 1.0; 2) have a current efficiency at least 85%; 3) have a current density of at least 0.6 ampere/in.sup.2 ; 4) be capable of operating at near 0.degree. Centigrade, and 5) allow incorporation of chlorine generating anode into the cell. Methods disclosed in the prior art cannot meet all of these requirements simultaneously. A new electrosynthesis process, one specific for the needs of generating basic hydrogen peroxide for the chemical oxygen-iodine laser, is required.